He's an indoor cat, so recent tick exposure is not a possibility. But he was a stray cat before, so who knows what he was exposed to... but that's actually a good point.
When I first brought him in, I had him tested for FeLV and FIV, but his test came back negative. I heard it can lie dormant for years though (another rescue from my area apparently had dormant FeLV, neg results initially) - maybe I should get another FeLV test.
At this point, I just want to know why this is happening and how I can stop the progression... I'm really hoping he will come home.

Once again, to everyone who has commented thus far - thank you so much for your support and insight. I cannot tell you how much I appreciate it.